The invention relates to a method of manufacturing tubes. In the method, a mixture of a powdered tube material and a liquid binder is introduced into a tubular mold. The mold has a geometry corresponding to the geometry of the tube to be formed. The tubular mold is rotated about its longitudinal axis while the powder-binder mixture is deposited on the inner wall of the mold so that excessive binder is removed. The tubular green body is then further processed.
The invention further relates to a device for performing such a method.
A device of the kind described alone is described in British Patent 682,580. The device is used to manufacture porous glass tubes, for example to produce laboratory filters, in which the pore distribution and pore size among the sintered glass particles is as uniform as possible, and in which the pores are interconnected.
For this purpose, the powder-binder mixture is a suspension of glass powder with a grain size which is as uniform as possible. The glass powder is mixed with a binder which dispenses the solid particles, for example an aqueous glycerin solution. Wetting agents and anti-foaming agents are also added. The ingredients are proportioned to provide the desired porosity, and they are provided in a centrifuge to form a loose structure of deposited solid particles on the inner wall of the mold.
Such a method is not suitable when solid particles are to be deposited on the inner wall of a tubular mold by centrifugal forces and a packing density which is as great as possible should be obtained.
A method of manufacturing glass bodies which are to be used as so-called preforms for the manufacture of optical fibers is described in German Patent 3240355 (corresponding to British Patent Application 2,129,418). This method also centrifugally deposits glass particles, but not from a suspension. Instead the particles are deposited dry on the inner wall of the tubular mold.
In this method, the dry solid particles form a loose structure and are held against the inner wall of the supporting member by centrifugal force during molding. The loose structure must then be formed into a self supporting structure by heating and/or by providing curing adhesives. The structure must then be compressed to form a fine-pored solid body, since the density obtained by molding does not satisfy the requirements for an optical fiber preform.
Disadvantages inherent to this known method are that the loose structure must be made self supporting, and that the solid body must be compressed to obtain the density required for an optical fiber preform. The compression of the solid body, in addition to requiring an additional process step, has the disadvantage that an intentional refractive index variation initially produced could be adversely altered.
Since the powders are centrifuged dry, problems of homogeneity may also occur. Due to electrostatic charges, uniform sedimentation and cohesion of the formed body is impeded. A further disadvantage may be that the loose structure exhibits nonhomogenous shrinkage behavior. This may lead to delamination of individual layers.